Flare behaviour in single and binary F and G stars – I. Flare OPEA models and saturation

ABSTRACT In this study, we analyse a data set comprising 8949 flares detected from both ground-based spectroscopic and space-based photometric observations of 51 sources, where 40 of them are single stars and the rest 11 of them are binary/multiple systems, spanning a range of evolutionary stages within the F- and G-type spectral classes. The primary aim is to investigate how the lower and upper bounds of the energy radiated during flares vary with stellar spectral type and evolutionary status. Furthermore, we explore how the height of magnetic loops, where flares originate within the stellar atmosphere, and the geometry of associated active regions depend on both spectral type and, in particular, evolutionary phase. We find that the ${\rm Plateau}$ value, derived from the OPEA (One Phase Exponential Association) models constructed for each source and regarded as an indicator of flare energy saturation, exhibits a clear dependence on stellar temperature, increasing towards cooler stars. Among G-type stars, however, the ${\rm Plateau}$ value shows no significant variation with stellar rotational period. Regarding flare time-scales, the total duration of the longest observed flares tends to increase with decreasing surface gravity ($\log g$). A decline in $\log g$ is associated with an increase in the total flare duration, suggesting an evolutionary influence on the flare process. Moreover, the ${\rm Half\hbox{-}time}$ parameter, which represents the minimum total duration at which the flare energy of a star reaches its saturation level, also varies systematically with evolutionary status.